Mr Johnson, a former mayor of London, said people “want to see officers on the streets” and he backed increased use of stop-and-search tactics as a way to tackle knife crime.

Speaking on LBC Radio, Mr Johnson was asked whether police cuts had contributed to the problems on London’s streets.

He said: “Keeping numbers high on the streets is certainly important. But the question is where you spend the money and where you deploy the officers.

“One comment I would make is that I think an awful lot of the money, an awful lot of police time, now goes into these historic offences and all this malarkey – £60 million I saw was being spaffed up the wall on some investigation into historic child abuse and all this kind of thing.

“What on earth is that going to do to protect the public now? What the people want is to see officers out on the streets doing what they signed up to do.”

A NSPCC spokesman said: “Bringing child abuse perpetrators to justice is not a ‘malarkey’ and such crass language is an affront to victims who have suffered in silence for decades.

“Investigation of historic allegations is an important part of creating a culture where we can talk more openly about abuse.

“These investigations will also help us all to learn lessons so that as a society we do not repeat the mistakes of the past.”

Labour MP John Mann said: “Boris Johnson says investigating child abuse was a waste of money. Try telling that to my constituent whose rapist got 19 years after we pressured for the case to be reopened.”

In a message aimed at Mr Johnson, shadow police minister Louise Haigh said: “Could you look the victims in the eye and tell them investigating and bringing to justice those who abused them, as children, is a waste of money? You shameless, dangerous oaf.”

Labour Party chairman Ian Lavery said: “If Boris Johnson has even a little bit of decency he will now apologise to the victims and families of those who have suffered.”

Theresa May’s official spokesman said the Prime Minister had not heard Mr Johnson’s comments.